This invention relates to an improvement of a scanning device described in FIGS. 3, 4 and 5 of U.S. Pat. No. 3,547,512 to Stephen C. Baer, the inventor of the present invention. An aim of that prior art scanning device was to provide, in a scanning microscope, a mechanism for scanning an optical assembly about an axis located at a substantial distance from the assembly, while minimizing the transmission of vibration to the stationary microscope frame or "base frame" during scanning. Because the moment of inertia of a rotating mass is proportional to the square of its distance from the axis, the torque which a scanner must impart on a scanned element to reverse its direction of rotation, and therefore the countertorque imparted to the scanner with each reversal, is critically dependent on the location of the scanning axis with respect to the scanned element. Since the countertorque imparted to the scanner with these reversals is a source of vibration transfer to the base frame, the potential to transmit vibration to the base frame is also critically dependent on the location of the scanning axis. In the common scanning application where the scanned element can be centered on the scanning axis, the greatest distance from that axis to any part of the scanned element is one half of the length of that element in the dimension perpendicular to the axis, and most mass in the scanned element is much closer than that distance, resulting in a relatively small moment of inertia for a given scanned element mass. In contrast, the optical assembly described in U.S. Pat. No. 3,547,512 required scanning about an axis at a considerable distance from the center of mass of the scanned assembly, specifically, by a distance substantially larger than the largest dimension of the scanned assembly.
To help minimize vibration transmission to the base frame in the scanning device described in U.S. Pat. No. 3,547,512, a counterrotating counterbalance mass was mounted on the same axis of rotation as the axis of rotation of the scanned optical assembly, in such a manner that when the scanned optical element changed direction during oscillation, the resultant countertorque was transmitted to this counterbalance rather than to the base frame. Impulses of momentum during these reversals were applied to the base frame from the scanned assembly and from the counterbalance in equal magnitudes from opposite directions, thus cancelling each other. Furthermore, in the scanning device described in U.S. Pat. No. 3,547,512, the external power to sustain the oscillation was transduced into a torque between the two counterrotating elements, rather than as a torque between either of these elements and the base frame, again reducing transmission of vibration to the base frame.
The scanning device described in U.S. Pat. No. 3,547,512 was designed under the assumption that when employing a counterrotating counterbalance to reduce transmission of momentum to the base frame during scanning, this aim of reducing transmission of momentum would be best realized by minimizing the moment of inertia of the scanned assembly. Therefore to minimize surplus mass on the arm holding the mirror-slit assembly for rotation about the axis, the axis of rotation was placed at one end of this arm, and the mirror-slit assembly at the other end. In spite of these efforts, however, it was found that with this arrangement, no amount of adjustment could reduce vibration transmission to the base frame sufficiently to allow enclosure of the scanning device within the body of a compact microscope.
In the present scanning application, several additional factors besides the increased moment of inertia of the scanned element work against successful resolution of this vibration problem. Firstly, because the scanner is mounted on a microscope, which can experience significant image degradation with a vibration even in the submicron amplitude range, vibration must reduced much more than with many other scanning applications. Moreover, because the aim was to develop a compact microscope, there were limits on the use of sheer massiveness of the stationary microscope base frame to reduce vibration. Also, because the scanned element is in fact an assembly of optical elements, not only must the mass of these individual elements be scanned, but in addition the scanning load includes the mass of the structure required to insure that these elements remain in precise alignment during the stress of scanning, so that not only is the moment of inertia large in proportion to the scanned mass, but the scanned mass itself is also quite large, these two factors together generating an extremely large moment of inertia. Also, to produce a constant image brightness over the image field, it was considered important to maintain a linear scan velocity over the image field, ruling out simple sinusoidal scanners, which by eliminating the required periodic collisions, avoid much of the vibration generation of linear scanners such as described in U.S. Pat. No. 3,547,512. Finally, because in this type of scanning microscopy (confocal microscopy), image quality is almost always considerably improved by a brighter image, it is desirable to have as much of the scan time as possible spent during active imaging as opposed to dead time at the extremes of the scan, and consequently the collisions were required to transfer a maximum of force in the least time, again accentuating the problem of vibration transmission. In view of these considerations, none of the prior art scanning devices now described, by themselves or in combination, suggest a solution to this vibration problem.
U.S. Pat. No. 3,453,464 to Baker describes an oscillating system of two counterrotating masses mounted on a common axis of rotation which passes through the center of mass of each of these masses. When the scanning axis of a scanned element passes through the element or assembly it may be possible to locate the element such that its center of mass and the scanning axis coincide. U.S. Pat. No. 3,952,217 to Rawlings, for example, describes such an arrangement where the scanned mass could have been positioned so that the center of mass of the scanned element coincided with the rotation axis, though no explicit mention was made of such coincidence. Unfortunately, when the scanning axis must be external to the scanned optical element, this axis cannot pass through its center of mass.
U.S. Pat. No. 3,66,974 to Dostal, U.S. Pat. No. 3,642,344 to Corker and U.S. Pat. No. 4,919,500 to Paulsen describe a class of torsion bar resonant scanners with oppositely rotating masses, which are enlarged segments along the axis of a common spring metal element, excited into a mode of resonance where these segments rotate in opposite directions. These scanners lack bearings, a desirable feature from the point of view of the high frequency scanning of low mass balanced loads for which these scanners were designed, but making them poorly suited for the present scanning application. Bearings in a counterrotating scanning device, by insuring that the axes of the two oppositely rotating masses remain in strict alignment during scanning, can permit scanning of off-center loads such as the mirror-slit assembly in U.S. Pat. No. 3,547,512 because the bearings join and cancel forces generated by the active scanned mass against corresponding forces from the counterrotating mass. Such uncancelled forces would be particularly troublesome in these torsion rod scanners because of the ease of exciting unwanted modes of vibration, thus a centered load may be an intrinsic requirement. Each of these scanners is powered by torque applied between the base frame of the scanner and at least one rotating element, an arrangement which can allow transmission of vibration to the base frame. Furthermore, when such a bearingless scanner is used in any orientation other than strictly vertical for the central axis, and where relatively very large scanned masses are required, as in the present application, gravitational loading of the scanned element and the counterrotating mass can cause bending of the central shaft, resulting in lack of perfect coincidence between the axes of the oppositely rotating masses, further reducing the effectiveness of the cancellation of vibration.
U.S. Pat. No. 3,846,784 to Sinclair describes a device for reciprocally scanning an optical element (a digital display assembly of light emitting segments) about an axis located at a significant distance from such element, and counterrotation of a counterbalance element about the same axis. FIG. 3 of this patent shows an embodiment where part of the electrical motor mechanism is directly mounted on a rigid superassembly containing the scanned element, and such an arrangement would create the possibility of locating the scanning axis through the center of mass of the scanned superassembly even though such axis was external to the scanned optical element. However because in this FIG. 3 of U.S. Pat. No. 3,846,784, all the mass of such motor mechanism attached to the scanned element is shown to be on only one side of the line passing through the rotation axis and through the center the scanned element, the center of mass of the superassembly consisting of the scanned element, the motor element and the connecting member between these elements, cannot be coincident with the rotation axis. Therefore, for reasons which will be become apparent, this type of scanner will also be subject to the same type of residual vibration as the scanner described in U.S. Pat. No. 3,547,512.
(The scanned element will often be referred to here by the term "scanned assembly" in case this scanned element contains a plurality of elements which remain motionless with respect to each other during scanning and which cooperate with each other to perform the particular task for which scanning is required, thus, in the case of the mirror-slit assembly, the two slits and the mirror cooperate with each other to perform the particular optical task for which scanning is required. The term "superassembly" will refer to the scanned element or assembly in combination with all other elements which rotate as a solid body with the scanned element, including those elements which do not cooperate with the scanned element in the performance of its particular task, even though they may be necessary for the scanning of the scanned element. Examples of elements not cooperating with the scanned element in the performance of its particular task include elements which hold the scanned element in place in relation to the scanning axis, motor means to power the oscillation, bearings to constrain the rotation of the scanned elements, position measuring means to measure the scan angle, and counterbalances which do not cooperate with the scanned element in the performance of its particular task.)
Another prior art patent describing scanning of an optical element about an axis at a significant distance from its center of mass is U.S. Pat. No. 4,902,083 to Wells. Vibration transfer to the supporting structure was reduced by the presence of a counterrotating counterbalance which rotated about an axis parallel to the rotation axis of the scanned optical element. Because the device described in that patent had to be small and light weight enough to be mounted on a person's head, it was impractical to control residual vibration transmission simply by increasing the mass of the base frame. It was acknowledged in that patent that such a design inherently results in the transmission of some vibration to the base frame structure. Nevertheless the only solutions proposed to this vibration problem in the patent were to ignore the vibration or to attempt to isolate it by means of a compliant suspension, rather than to eliminate the vibration at its source which is the principal aim of the present invention.
Besides transmitting vibration to the base frame during scanning, when used to scan a relatively massive optical element about an axis at a substantial distance from the element, these prior art scanner designs require custom construction of the electromechanical and other components of the device, adding to the expense of the scanning device. In these prior art designs, the counterrotating member is of a size comparable to the active scanned member, and when the scanning axis must be a relatively large distance from the scanned optical element, the resultant large size of the scanner can pose a serious problems when the overall dimensions of the instrument should be as small as possible.